Electronic devices, which are “mobile” or portable, are being used today for various commercial or personal applications. Examples of such devices include mobile phones, notebook computers, personal digital assistants (PDA) or portable music players. The afore-mentioned devices are usually powered by a rechargeable battery, which has to be recharged periodically to keep the device operational. For recharging or for supplying external power, each device normally has a dedicated power supply unit, which has to be connected to the device and to a wall power outlet. Since most devices are not compatible with each other, a dedicated power supply unit is needed for each device.
Due to the fact that the total number of such devices used increases, wireless power sources have been developed, which reduce time and effort of the recharging process. US 2007/0182367A1 discloses such a wireless power source and charging system. A base unit is provided, having a surface for accommodating one or more mobile devices to be recharged. Several inductor coils are provided below the surface for generating a magnetic field, which induces a current in a corresponding coil of each mobile device. Using this current, a rechargeable battery in the mobile device can be recharged easily and without the process of connecting a custom power supply unit to each of the devices to be recharged.
Although the known system allows to wirelessly recharge such a device, the inductor coils in the base unit generate a rather large magnetic stray field which causes problems in EMC-sensitive environments and may cause unwanted interference with further electronic devices.
Accordingly, it is an object of this invention to provide a wireless power transmission system and method for operating such system, in which the magnetic stray field is reduced.